The device, referred to hereinafter as the three-knife trimmer, is responsible for cutting the provided printed products, i.e. mostly book blocks and/or booklets, on the three open sides. This is carried out by the book block or the booklet (referred to hereinafter only as book block) being clamped, when stationary, between pressure bars or pressure plates and three cutting apparatuses trimming the three aforementioned sides of the book block. The cutting apparatuses can be in the form of counter blade units, in which two blades cut relative to one another like scissors, or as blade units having cutting bars, in which a blade cuts against a plastics bar and slightly penetrates the plastics bar when in the final position.
In three-knife trimmers of this type, the head and foot are generally cut in a first phase and the front in a second phase. However, this order is not compulsory and can also be the other way around. It is also possible for only the head and foot or only the front to be cut on the book block, as is required for example for the production of English booklets. There are three-knife trimmer designs in which the book block remains stationary between the first cutting phase (e.g. head and foot cut) and the second cutting phase (e.g. front cut), and there are designs in which the book block is transported between said phases.
Three-knife trimmers are known in which, for the cutting, the book blocks are pressed and held between press rams and cutting cartridges. When the press ram is raised, the cut book block is conveyed away and the book block to be cut next is introduced. The book block is moved into position by a centring device and then clamped by the press ram travelling therebelow. The blades move against the book in a swinging cut and cut the open sides. Once all the sides have been cut, the press ram is raised and the next work cycle can begin. This three-knife trimmer design is not capable of a quick format change. The press ram and the cutting cartridge are tailored to the format to be processed and can only be exchanged by stopping the machine.
DE 10 2011 105 253 A1 discloses a three-knife trimmer in which the blades cut against cutting bars or counter blades, the book block being held next to the blades by pressure bars when the head, foot and front are cut. In the gap between the cutting bars or counter blades on the one hand, which gap varies depending on the format, and the space between the pressure bars on the other, there is arranged a plurality of zigzag support ribs for supporting the book block during the cutting. By means of a three-knife trimmer of this type, good quality cutting can be achieved because the book block is pressed or supported by the pressure bars and the zigzag support ribs during the cutting.
However, it has to be borne in mind with this solution that the book block can get caught on the zigzag support ribs when being conveyed into and out of the cutting position. This fact is counteracted in DE 10 2011 105 253 A1 by the transport system consisting of a lower belt and an upper belt, the two belts for transporting the book block being driven more or less together relative to one another so that the book block cannot get caught on the support ribs of the bed plane or pressing plane. However, when the book block is transported thus, only one book block can ever be transported in the transport system if the book blocks have different thicknesses. This limits the permissible thickness difference from book block to book block since there is a plurality of book blocks in the transport system. In particular, in three-knife trimmers, in which the cutting is carried out in two stations, problems arise when using the described transport system to transport the book blocks that vary greatly in thickness. In the case of book blocks that vary greatly in thickness, it is thus necessary to only ever transport one book block within the transport system. In such a three-knife trimmer, however, this limits the capacity, i.e. the three-knife trimmer can only be operated with a low number of cycles (low output).
DE 10 2011 105 253 A1 sets out further three-knife trimmers that define prior art. None of them, however, makes it possible to satisfy the requirements of low changeover time combined with the requirement of high cut quality.
EP 1 504 860 A1 discloses a three-knife trimmer in which the book blocks to be trimmed are gripped by a positioning device and supplied to the cutting apparatuses by a feed apparatus. A plurality of spaced-apart cutting apparatuses are provided, in which the book blocks are positioned one behind the other by the feed apparatus for a side cut in each case. In each cutting apparatus, a side cut is carried out on the positioned printed product. The oriented book blocks are moved from the positioning apparatus into a transition position by an intake gripper by means of a linear stroke, the orientation of the book blocks not being changed. The book blocks are fed by a multiple epicyclical gearing. To position the book blocks in the cutting apparatuses, adjustable control links are provided. The device allows for simple and quick changeover to different formats. Each cutting apparatus consists of a lower blade fixed to the frame, and an upper blade to which a pressure plate is coupled by means of a guide and a pneumatic cylinder. Prior to the cutting, the pressure plate clamps the bookblock between the pressure plate and the stationary lower blade. In the process, the bookblock is not pressed by the pressure plate, the lower blade and the feed apparatus over a large surface area, but rather just in the cutting region. The regions of the book block that are not pressed tend to “sag” and can thus lead to an unsatisfactory cut quality. This is the case in particular when soft and/or thin paper is used for the book blocks.
JP 2012-218114 A discloses a three-knife trimmer which can process different book formats one after the other and in which the outer sides of the book are not damaged. A mounting unit for gripping the rear side of the printed product is attached to a moving part, the mounting unit comprising a reference surface for positioning the spine of the printed product. By means of a positioning actuator, the moving part is moved in a vertical plane by a controller and positioned, in each case in the correct manner for the format, on the three open sides of the printed product for the cutting processes so that the cutting blades moving in the horizontal direction can cut the printed product. The printed product is oriented on the reference surface of the mounting unit and on a vertical bearing surface, which allows the controller, together with the format data, to approach the positions required for each cut and to position the printed product correctly for the cutting.
The disadvantage of this three-knife trimmer is the limited possibility of varying the book format. Since the one mounting unit holds the printed product for all three cuts, the mounting unit has to be significantly smaller than the smallest printed product to be processed. If the printed product has a significantly larger format, the plates that are also used for supporting the printed product during the cutting have to be additionally provided with a large recessed region. However, a large recessed region has a negative effect on the cut quality.
For the front cut, the mounting unit can enter the recessed region of the plates to a greater or lesser extent. The printed product is then only properly supported when the mounting unit is inserted far into the recessed region of the plates. So that the printed product can be sufficiently supported without exchanging the plates, the three-knife trimmer can only be used to process books with a small difference in width.